Systems and method for high data rate ultra wideband communication

ABSTRACT

A high data rate UWB system implements a frame structure that uses a connected set of m-sequences comprising the lowest possible cross-correlation and perfect, or near perfect autocorrelation. Each m-sequence can be used to identify a different piconet. A very efficient code matched filter can then be used to decode the frames and achieve synchronization with a piconets.

RELATED APPLICATION INFORMATION

This application claims priority under 35 U.S.C. 119(e) to U.S.Provisional Application Ser. No. 60/700,766 entitled “Systems andMethods for High Data Rate Ultra Wideband Communications,” filed Jul.20, 2005, which is incorporated herein by reference in its entirety asif set forth in full.

BACKGROUND

1. Field of the Invention

The embodiments described herein are related to Ultra Widebandcommunication systems, and more particularly to efficient communicationwith reduced processing overhead in a Ultra Wideband communicationsystem.

2. Background

FIG. 1 is a diagram illustrating an exemplary frame structure 102 usedin Ultra Wide Band (UWB) communication systems. As can be seen, frame102 comprises a preamble 104 separated from a packet payload, or dataportion 108 by a header 106. Preamble 104 can comprise a packet syncsequence (SYNC) 110, a start frame delimiter (SFD) 112, and a channelestimation sequence (CE) 114. In many conventional systems, SYNC 110 iscomprised of a plurality of codes 116. SYNC 110 is used to detect thebeginning frame 102 and allows a device to lock to a specific piconet.Each piconet uses a different code 116 for SYNC 110. SFD 112 is used toindicate the end of SYNC 110 and CE 114 is used to estimate multi-pathfading for the channel being received. SFD 112 also comprises ofplurality of codes 118. CE 114 also comprises a plurality of codes 120.

Thus, in a UWB system a piconet could be configured to transmit data 108within frame structures 102. UWB devices operating in range of thepiconet can then be configured to receive packets 102 from the piconetand synchronize their receivers to the transmitter in the piconet bydetecting and correlating SYNC 110 within frame 102. Because the devicehas no knowledge or where within SYNC 110 it begins receiving frame 102,SFD 112 can be included in order to indicate that the end of SYNC 110 isbeing received. Accordingly, when SFD 112 is detected by the receiver,the receiver can be configured to stop trying to synchronize itself withthe transmitter and, if synchronization is achieved, prepare to receivedata 108. CE 114 can be used by the receiver to estimate the effects ofmulti-path fading for the received signal.

The receiver front-end manipulates the received data and converts it toa digital signal that can be passed to a digital back-end within thereceiver. This digital back-end can comprise the components necessary todetermine, for example, whether the receiver is synchronized with thetransmitter. FIG. 2 is a diagram illustrating an exemplary portion 200of a digital back-end receiver configured to determine whether or notthe receiver is synchronized with the transmitter in the piconet.Receiver portion 200 can comprise a code match filter 202, which can beconfigured to perform a coherent filtering, e.g., de-spreading, of thereceived signal. The coherent filtering, or de-spreading processtypically results in a processing gain being applied to the receivedsignal at the output of code match filter 202.

The processing gain can be approximately equal to the number of chipsincluded in code 116. Thus, for example, if code 116 comprises 127chips, then a processing gain of 127 can be achieved by code matchfilter 202.

The output of code match filter 202 is then passed to a squarer 204,which can be configured to take the power of the signal at the output ofcode match filter 202 and perform a squaring operation. The output ofsquarer 204 can then be passed to accumulator 206, which can beconfigured in non-coherently accumulate the power of the output ofsquare root 204.

The output of accumulator 206 can then be passed to a decision block208, which can be configured to compare the accumulator output to athreshold and decide whether the piconet signal is in fact present. Ifthe receiver is sufficiently synchronized with the piconet, then thepower at the output of accumulator should be sufficient to surpass thethreshold indicating that the piconet signal is in fact present. Ifsignals from the piconet are not being received, or if the receiver isnot synchronized with the transmitter in the piconet, then the power atthe output of accumulator 206 should not be sufficient to surpass thethreshold and decision block 208 should indicate that the piconet signalis not present.

FIG. 3 is a diagram illustrating an exemplary code match filter 202 thatis often used in conventional receiver portions 200. Such a conventionalcode match filter 202 typically comprises a plurality of filter taps302, e.g., 127 filter taps 302 for a code 116 comprising 127 chips.Filter taps 302 are separated by delay blocks 304 and are coupled withthe inputs of multipliers 306, which can be configured to multiply thetap input by a coefficient, as is well known. The output of multipliers306 can then be provided to an adder tree 308 where the outputs arecombined into a single signal.

Such conventional code match filter designs suffer from several problemsthat can limit efficiency and over-all performance. For example, onechallenge presented by such a conventional code match filter design isthat the circuit has to run at a very high speed approximately equal tothe bandwidth of the system. As is understood, UWB systems haveextremely wide bandwidths, e.g., 1.5 GHz. Thus, code match filter 202must be configured to run at a speed of approximately 1.5 GHz. Thispresents many design problems. One problem is that the circuits includedin code match filter 202 can have difficulty running at such highspeeds. Further, operation at such high speeds can present increaseerror tolerances, and consume much larger amounts of current. Anotherproblem with such a conventional code match filter 202 is that it tendsto be highly complex. For example, adder tree 308 requires manysub-adders, e.g., as many as 126-sub adders for a code 116 of length127. This increased complexity increases error tolerances, increases thearea required to implement the code match filter, and can also increasecurrent consumption.

It will also be understood that at least in the United States, UWBsystems are configured to operate in what is referred to as theunlicensed spectral band. Because many systems, including manygovernment and safety systems, operate in the unlicensed band, theFederal Communications Commission (FCC) regulates the transmit power fordevices operating in the unlicensed band. The regulations in somerespects attempt to limit the peak output power. It will be understoodthat limiting the peak output power will also limit the transmit rangeof a device. It will also be understood that it is often preferablemaximize the transmit range for effective and efficient communication.Accordingly, the FCC's regulations can be at odds with performanceobjectives.

It will be understood that it can be preferable to have a flat transmitspectrum. A flat transmit spectrum allows for the maximum transmit powereven in the face of the FCC's regulations. It will also be understoodthat the above issues can limit the effectiveness and efficiency ofconventional UWB receiver designs and prevent them from meeting therequirements of the specific implementation, e.g., in terms of batterylife and size.

SUMMARY

A high data rate UWB system implements a frame structure that uses aconnected set of m-sequences comprising the lowest possiblecross-correlation and perfect, or near perfect autocorrelation. Eachm-sequence can be used to identify a differnet piconet A very efficientcode matched filter can then be used to decode the frames and achievesynchronization with a piconets.

In one aspect, the m-sequences can be 127 chips in length.

In another aspect, different codes can be used to spread data beingtransmitted in order to achieve different operational data rates.

In still another embodiment, the spreading codes can be selected andimplemented in such a manner as to achieve a flat, or nearly flattransmit spectrum.

These and other features, aspects, and embodiments of the invention aredescribed below in the section entitled “Detailed Description.”

BRIEF DESCRIPTION OF THE DRAWINGS

Features, aspects, and embodiments of the inventions are described inconjunction with the attached drawings, in which:

FIG. 1 is a diagram illustrating an exemplary frame structure for use ina conventional UWB system,

FIG. 2 is a diagram illustrating a portion of a receiver used in aconventional UWB system,

FIG. 3 is a diagram illustrating an exemplary code matched filter usedin the receiver of FIG. 2.

FIG. 4 is a diagram illustrating examples of a frame structure that canbe used in a UWB system in accordance with one embodiment;

FIG. 5 is a diagram illustrating an example code matched filter that canbe implemented in a UWB system that used the frame structure of FIG. 4in accordance with one embodiment;

FIG. 6 is a diagram illustrating a portion of a receiver that caninclude the code matched filter of FIG. 5 in accordance with oneembodiment;

FIG. 7 is a diagram illustrating the spectrum produced when data isspread using the code [1.1];

FIG. 8 is a diagram illustrating the spectrum produced when data isspread using the code [1,-1]; and

FIG. 9 is a diagram illustrating the spectrum produced when the codes[1.1] and [1,-1] are used in combination to spread data in accordancewith one embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Certain embodiments described below are directed to high data rate UWBsystems comprising a plurality of piconets. As will be understood, thepiconets comprise a Service Access Point (SAP) that is configured tocommunicate with remote communication devices and allow access to thesystem in much the same way that base stations communicate with devicesand allow access to the network in a cellular communication system. Oneor more of the piconets can have overlapping boundaries and each can usethe same spectrum for communication. Accordingly, remote communicationdevices operating within the system should be capable of identifying andsynchronizing with a particular piconets. Further, the system should bedesigned such that adjacent piconets do not interfere with each other.It can also be preferable for the system to be designed so as tomaintain the maximum possible output power in order to increase therange.

For example, the UWB system can comprise a network range for eachpiconets of 25 meters with a network bandwidth of approximately 1.5 GHz.The chip rate for the system can then be defined as 1/bandwidth, whichis equal to 0.666 ns. Data to be communicated on the downlink, i.e.,form the piconets or SAP to the remote device, can then be organizedinto frames. The frames can use certain codes or sequences to allow aremote device to synchronize with the respective piconets. The codeswill comprise a certain number of chips. As explained below each chipcan represent one or more bits of information. The codes used forsynchronization should be sufficient to allow a remote device tosynchronize with the piconets even in the face of multipath.Accordingly, the code length should be defined as:Δt=Δr/c, whereΔt=the delay spread,Δr=the distance between the first and last multipath, andc=the speed of light.

Using the example parameters provided above, Ar is equal to 25 meters,which provides a delay spread (Δt) of 83 ns or 127 chips. Thus, the codelength should be 127 chips. It will be understood that the parametersprovided are by way of example only and that other parameters can beused in accordance with the systems and methods described herein.

It will be further understood that the term remote device refers to adevice that is remote form the SAP. Such a remote device can be a mobileor fixed device.

FIG. 4 is a diagram illustrating an example frame structure 400 that canbe used for the downlink configured in accordance with one embodiment ofthe systems and methods described herein. Frame 400 comprises a preamble402 separated from a packet payload, or data portion 406 by header 404.In the example of FIG. 4, data portion 406 can comprise various fieldsdepending on the embodiment. In one example embodiment, data portion 406a can be included in frame 400. Data portion 406 a comprises data field408 separated by zeros as illustrated. Other embodiments, for example,can include a data portion 406 b, however. Data portion 406 b includesdata fields 408 separated by channel estimation sequences (CE) 410.

In data portion 406 a, zeroes are included in order to let nay multipathdecay, before decoding the next data field 408. In data portion 406 b,CEs 410 are included to improve channel estimation. In other words, aknown pattern can be used for CE 410, which can then be used by a remotedevice for improved channel estimation.

As with frame 102, preamble 402 can comprise a SYNC field 412, and SFD)field 414, and a Cl field 416. Unlike frame 102, however, SYNC 412 cancomprise the same sequence (SO) repeated a certain number of times. SFD414 can also be configured from the same sequence; however, the inverseof the sequence (-S₀) can be used as illustrated. Cl 416 can also beconstructed from the same sequence (S₀). In one embodiment, sequence(S₀) is an m-sequence, specifically an m-sequence of length 127. Evenmore specifically, maximum length, or m-sequences can be selected thatform a connected set of sequences, which means that the sequencesselected exhibit very low cross correlation and almost perfectauto-correlation.

As explained in co-pending U.S. patent application Ser. No. 11/425,964entitled “Systems And Methods For Generating a Common Preamble For UseIn a Wireless Communication System,” filed Jun. 22, 2006, and which isincorporated herein by reference as if set forth in full, the use ofm-sequences with perfect autocorrelation and low cross-correlation canbe used to limit interference between piconets.

For example, in embodiments that used m-sequences of length 127, it canbe shown that there are 6 sequences that meet the requirements for aconnected set described herein. In other words, it can be shown thatthere are 6 m-sequences of length 127 that have the lowest possiblecross-correlation, in this case about 17. As a result, each of these 6sequences can be assigned to a different piconets, which enables systemsthat include 6 piconets operating within overlapping range of eachother.

Moreover, use of a matched set of m-sequences in the manner describedand illustrated in FIG. 4 allows the use of a very efficient code matchfilter design. FIG. 5 is a diagram illustrating an example code matchfilter 500 configured in accordance with the systems and methodsdescribed herein. As can be seen, code match filter 500 can beconfigured to receive a serial stream of data bits (x_(n)). The serialdata can be input into a serial-to-parallel converter 502, which can beconfigured to convert the serial data into a parallel data asillustrated. As result, the data can be processed at a much lower speedthan derail data stream (x_(n)). This reduces the complexity andprocessing overhead required.

The parallel data can then be passed to a permuter 504 configured topermute the parallel data, which can then be passed to a fast Walshtransform block 506. Each parallel data stream will have its ownpermutation. The data is permuted so that it can be process via a fastWalsh transform. The output of fast Walsh transform block 506 can thenbe provided to permuter 508, which essentially undoes the permutationperformed by permuter 504. The output of permuter 508 can then be passedto a parallel-to-serial converter 510, which can be configured toconvert the parallel data of the output of permuter 508 into a serialdata as illustrated.

It should be noted that code match filter 500 can be implemented in avery low complexity design relative to conventional code match filters,such as those described above, which decreases cost, decreases sizerequirements, and can increase efficiency.

FIG. 6 in the diagram of a receiver portion 600 configured to determinewhether or not a receiver is synchronized with a piconets transmittingframes 400 that includes an efficient code match filter, such as codematch filter 500, in accordance with one embodiment of the systems andmethods described herein. Receiver portion 600 can include a coherentaccumulator 602 configured to accumulate M periods of the m-sequence.For example, M can be equal to 4 and thus coherent accumulator canaccumulate 4 periods of the m-sequence. By accumulating M periods of them-sequence, the speed of the circuitry needed to process frame 400 canbe reduced by a factor M. For example if M is equal to 4 then theprocessing speed required of the circuitry can be reduced from, e.g.,1.5 GHz to 0.375 GHz. In addition, interference is reduced, and noiserejection is increased, by the same factor, e.g., 4.

The output of coherent accumulator 602 can then be passed to efficientcode match filter 500, the output of which can be passed to squarer 604,then non-coherent accumulator 606, and finally decision block 608, whichcan be configured to compare the output of non-coherent accumulator 606with the threshold in order to determine whether signals from thepiconets are present.

Accordingly, implementation of potion 600 can greatly reduce overhead byreducing the processing speed at which portion 600 must operate.Further, interference is greatly reduced due to the use of them-sequences and the noise rejection provided by accumulating m periodsof the m-sequences.

In certain embodiments, spreading can also be implemented in order toincrease the range and interference rejection at the expense ofdecreasing the data rate. For example, in one embodiment, the data canbe spread by a factor of 2, e.g., with a BPSK code. This means thatthere will be 1 bit for every two chips. In other embodiments, thespreading factor can be 4, 8, and so on depending on the requirement ofa particular embodiment.

The spreading is achieved by coding each bit of data with multiplechips. For example, if the spreading factor is 2, then a 2 chip code isused to spread each data bit. If the spreading factor is 4, then a 4chip code is used to spread each data bit. It can be preferable toselect and implement the spreading codes so as to ensure that themaximum possible transmit power can be achieved. It will be understoodthat when a code is used to spread, the spectrum will not be flat. Thus,the output power may need to be reduced based on the FCC regulations.Proper selecting and implementation of the spreading codes can, however,counter act this.

For example, in the case of a spreading factor of 2, two spreading codescan be used, i.e., (1.1) or (1,-1). When spreading with a code of (1,1),the spectrum will not be flat, and consequently the power at thetransmitter has to be reduced in order to meet the FCC requirementimposed on the spectrum within a UWB system. FIG. 7 is a diagramillustrating the spectrum 702 that results when a code of (1,1) is used.Similarly, when a code of (1, -1) is used a non-flat spectrum isproduced. FIG. 8 is a diagram of the spectrum 802 that is produced whena code of (1, -1) is used. In each case the transmitter must be hackedoff by 3.3 dB, which reduces the range and at least partially defeatsthe purpose of using spreading codes in the first place.

In the systems and methods described herein, alternating codes can beused to spread the data within data fields 408. For example, the evennumbered fields 408, i.e., fields 0, 2, 4, etc., can be spread using thesequence (1,1), while odd numbered fields. i.e., 1, 3, 5, etc., arespread using the sequence (1, -1). The even data fields 408 will thenexhibit spectrum similar to that illustrated in FIG. 7 while odd datafields 408 will exhibit a spectrum similar to that illustrated in FIG.8; however, the over all spectrum for data field 406 within the frame400 will be a flat spectrum, such as spectrum 902 illustrated in FIG. 9.As a result, the power in the transmitter requires minimal reduction andbetter interference rejection can be achieved. For example, in oneembodiment the transmitter needs to be backed off by only 0.4 dB.

Again, guard intervals of no transmitted data, or CE field 410 can beincluded between data field 408. In embodiments that use a CE field 410,the need to track the channel can be eliminated, since CE field 410 canbe used to re-acquire the channel before each data field 408 isprocessed, which can lead to even greater efficiency.

It will be understood that similar techniques can be used for spreadingfactors of 4, 8, etc., in order to achieve a flatter spectrum.

While certain embodiments of the inventions have been described above,it will be understood that the embodiments described are by way ofexample only. Accordingly, the inventions should not be limited based onthe described embodiments. Rather, the scope of the inventions describedherein should only be limited in light of the claims that follow whentaken in conjunction with the above description and accompanyingdrawings.

1. A code matched filter for use in a Ultra Wideband (UWB) system,comprising: a serial to parallel converter configured to receive aserial data stream and convert the serial data stream into a pluralityof parallel data streams; a permuter coupled with the serial to parallelconverter, the permuter configured to permute each of the plurality ofdata streams; a fast Walsh transformer coupled with the permuter, thefast Walsh transformer configured to transform the plurality of permutedparallel data streams, a second permuter coupled with the fast Walshtransformer, the second permuter configured to permute the plurality oftransformed parallel data streams; and a parallel to serial convertercoupled with the second permuter, the parallel to serial converterconfigured to convert the plurality of parallel data streams to a serialdata stream.
 2. The code matched filter of claim 1, wherein the serialdata stream comprises n chips, and wherein the serial to parallelconverter is configured to convert the serial data into n parallel datastreams.
 3. The code matched filter of claim 1, wherein the serial datastream comprises a data rate of approximately 1.5 Ghz.
 4. The codematched filter of claim 1, wherein the serial data stream comprises acode, and wherein the code is one of a matched set of m-sequences. 5.The code matched filter of claim 4, wherein the m-sequences of thematched set exhibit perfect autocorrelation and the lowest possiblecross-correlation.
 6. The code matched filter of claim 5, wherein eachm-sequence in the matched set comprises 127 chips, and wherein the crosscorrelation is approximately
 17. 7. A receiver portion for use in a UWBremote device, the receiver portion comprising: a coherent accumulatorconfigured to receive data comprising a plurality of codes andaccumulate a certain umber M of the codes; and a code matched filtercoupled with the coherent accumulator, the code matched filtercomprising: a serial to parallel converter configured to receive aserial data stream from the coherent accumulator and convert the serialdata stream into a plurality of parallel data streams, a permutercoupled with the serial to parallel converter, the permuter configuredto permute each of the plurality of data streams, a fast Walshtransformer coupled with the permuter, the fast Walsh transformerconfigured to transform the plurality of permuted parallel data streams,a second permuter coupled with the fast Walsh transformer, the secondpermuter configured to permute the plurality of transformed paralleldata streams, and a parallel to serial converter coupled with the secondpermuter, the parallel to serial converter configured to convert theplurality of parallel data streams to a serial data stream.
 8. Thereceiver portion of claim 7, wherein the serial data stream comprises nchips, and wherein the serial to parallel converter is configured toconvert the serial data into n parallel data streams.
 9. The receiverportion of claim 7, wherein the serial data stream comprises a data rateof approximately 1.5 Ghz.
 10. The receiver portion of claim 7, whereinthe serial data stream comprises a code, and wherein the code is one ofa matched set of m-sequences.
 11. The receiver portion of claim 12,wherein the m-sequences of the matched set exhibit perfectautocorrelation and the lowest possible cross-correlation.
 12. Thereceiver portion of claim 11, wherein each m-sequence in the matched setcomprises 127 chips, and wherein the cross correlation is approximately17.
 13. The receiver portion of claim 7, further comprising a squarercoupled with the code matched filter, the squarer configured to squarethe power of the output signal produced by the code matched filter. 14.The receiver portion of claim 13, further comprising a non-coherentaccumulator coupled with the squarer, the non-coherent accumulatorconfigured to accumulate the output of the squarer.
 15. The receiverportion of claim 14, further comprising a comparer coupled with thenon-coherent accumulator, the comparer configured to compare the outputof the accumulator with a threshold.
 16. A transmitter configured tospread and transit data, the transmitter comprising: a frame generatorconfigured to generate a frame of data, the frame of data comprising aplurality of data fields; a spreader configured to spread the datafields using a plurality of codes, the codes selected so that thecombined spectrum for the spread data fields is substantially flat. 17.The transmitter of claim 16, wherein the codes are [1,1] and [1,-1]. 18.The transmitter of claim 16, wherein the data fields are separated by achannel estimation pattern.
 19. The transmitter of claim 16, wherein thedata fields are separated by zeroes.
 20. The transmitter of claim 16,wherein the frame comprises a preamble, the preamble comprising a syncfield, the sync field comprising a plurality of codes selected form amethod set of m-sequences.